A controversial county plan to build a regional sewer system in Malibu drew fire Tuesday from about 500 residents who alternately booed the plan's advocates and cheered its foes at the last scheduled public hearing before the Board of Supervisors votes on the project.

During the standing-room-only meeting, Malibu residents questioned the accuracy of a preliminary environmental impact report on the project, which includes a $38-million treatment plant in Corral Canyon.

"There is an obvious bias in favor of sewers and against not having a project," said Walter F. Keller, a mechanical engineer on the transportation committee of the Malibu Township Council, which represents 1,000 local families. "For instance, public acceptability is way lower than they think, and they don't weigh that properly."

Three Defeats

Malibu voters have turned down sewage bond issues three times since 1966, largely because they fear that a regional system will lead to overdevelopment in the environmentally sensitive area. Instead, they prefer to rely primarily on septic tanks and on six small private sewage systems.

But in April, 1985, the county Health Department declared that continued reliance on septic systems would be dangerous because nearly half of the existing septic tanks in Malibu have failed during the past decade.

The project can proceed despite public opposition, provided that four of the five county supervisors approve it, according to the report issued by county consultants from James M. Montgomery Consulting Engineers.

The county expects to open a sewage stystem that serves Malibu from Topanga Canyon on the east to Latigo Shores on the west by 1991. An expansion farther west to Point Dume would follow about 20 years later.

"They say it's all about health, but as I look around this room, I see a bunch of healthy people with suntans," said Geary Steffen, 62, who has lived in Malibu for 20 years. "My philosophy is, 'If it ain't broke, don't fix it,' and I submit that it ain't broke."

More than one resident who disputed the need for a public sewer system questioned Montgomery's objectivity in recommending the Corral Canyon project.

"I see it as a blatant conflict of interest since they may end up designing all or part of the system," said Lauren Achitoff, a member of the Corral Canyon Homeowners Assn.

There are no legal restrictions to prevent a firm that draws up an environmental impact report from bidding on whatever contract may result, said Brian Sasaki, an engineer with the Public Works Department.

Written Requests

Peter Ireland, a Malibu field deputy for Supervisor Deane Dana, said that if residents object, they can submit a written request to the county asking that Montgomery be prohibited from bidding if the project is approved.

During the hearing, Montgomery representatives presented a brief summary of their report and did not comment on residents' allegations or questions. However, repeated hisses and boos from the crowd prompted supervising engineer Jan Fahey to warn the audience, "If you don't quiet down, we're going to stop the meeting."

But residents vowed to derail the project, charging that the report underestimates its cost and the increased traffic, development and danger of slides that would result.

Nancy Ruggeri, 28, said she and her husband fear that the $1,600-per-month rent on their two-bedroom apartment will go up if their landlord is assessed the $13,000 to $26,000 estimated in the report.

"We've got the Johnny Carsons, but there are also a lot of people like me in Malibu who work for a living," said pharmacist Frank Basso, 55, who is vice president of the Malibu Township Council. "Some of us are going to have to move if this thing goes through."

According to the 1980 census, more than a third of the households in Malibu have incomes of less than $25,000 a year.

Increases Predicted

However, John Forry, 44, who has lived on Rambla Pacifico for 11 years, said the sewer will increase the value of homes in the area because it will decrease the chance of landslides.

"If we don't do sewering, the soil will get so saturated from the septic tank systems that we'll have more slides," Forry said. "Even if it costs $25,000 a lot, it will increase the value of each lot by $100,000, which makes it a good investment. But I think it's going to take a few crises to convince people."

The traffic congestion that would result from laying the sewer pipe along the ocean side of Pacific Coast Highway would be of crisis proportions, Keller said. He questioned the report's assessment of the traffic impact, which estimates that construction would be restricted to weekdays between 9 a.m. and 3 p.m. and that traffic would slow by only 10 miles per hour during those hours.

Lane Closure

Keller said two lanes would have to be closed, not just one, to accommodate the construction.

James L. McCullough, Caltrans senior operations engineer for the district including Malibu, said he is not familiar with the specifics of the proposed project.

"They'll have to apply for a permit before construction can begin, and at that time, we will evaluate their proposal," McCullough said. "But I think 10 miles per hour is an optimistic estimate since my general experience out there leads me to believe there will be stop-and-go traffic as a result."

County sewer official Brian Scanlon said there could be another public hearing, but none is scheduled.